Spring IOC加载全过程（转）

程序员文章站 2022-05-01 19:53:06

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本文章以ClassPathXmlApplicationContext为切入口，常用的还有ContextLoaderListener类。
Spring IOC加载全过程（转）

首先
**一般开始都是这样一句代码：

ApplicationContext context = new ClassPathXmlApplicationContext("classpath:application.xml");

意思是加载xml文件创建一个ApplicationContext 的Spring 容器。
那么就从new ClassPathXmlApplicationContext这个构造方法开始看吧。进入源码：
其实可以看到他是来到这里了

public ClassPathXmlApplicationContext(
		String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
		throws BeansException {

	super(parent);
	setConfigLocations(configLocations);
	if (refresh) {
		refresh();
	}
}

上面这个构造方法呢，其实就是new ClassPathXmlApplicationContext(“classpath:application.xml”) 真正开始的地方。开始之前大家先看看上面第二、第三张图“ApplicationContext 的继承图”和“BeanFactory的继承图”，ClassPathXmlApplicationContext 经过好几次继承才到 ApplicationContext 接口，了解一下ApplicationContext 的大体结构。

ApplicationContext 其实就是一个BeanFactory，

ApplicationContext 继承了ListableBeanFactory，这个ListableBeanFactory接口它可以获取多个bean，我们看BeanFactory接口的源码可以发现，BeanFactory的接口都是获取单个bean的
同时ApplicationContext 还继承了HierarchicalBeanFactory接口，这个接口可以在应用这起多个BeanFactory，然后将多个BeanFactory设置父子关系
ApplicationContext 接口中的最后一个方法：AutowireCapableBeanFactory getAutowireCapableBeanFactory() throws IllegalStateException; 他的返回值是AutowireCapableBeanFactory，这个接口就是用来自动装配Bean的
然后我们回到上面的 new ClassPathXmlApplicationContext(“classpath:application.xml”)构造方法。先看上面构造方法那个源码，setConfigLocations(configLocations);是根据提供的路径，处理成配置文件数组(以分号、逗号、空格、tab、换行符分割)，然后就到了重点的refresh(); 这个refresh();方法可以用来重新初始化ApplicationContext ，下面贴出来这个方法的源码：

@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}

可以看到这个方法里面调用了很多的方法，我们从开始说起：

首先是一个synchronized加锁，当然要加锁，不然你先调一次refresh()然后这次还没处理完又调一次，就会乱套了；
接着往下看prepareRefresh();这个方法是做准备工作的，记录容器的启动时间、标记“已启动”状态、处理配置文件中的占位符，可以点进去看看，这里就不多说了。
下一步ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();这个就很重要了，这一步是把配置文件解析成一个个Bean，并且注册到BeanFactory中，注意这里只是注册进去，并没有初始化。先继续往下看，等会展开这个方法详细解读
然后是prepareBeanFactory(beanFactory);这个方法的作用是：设置 BeanFactory 的类加载器，添加几个 BeanPostProcessor，手动注册几个特殊的 bean，这里都是spring里面的特殊处理，然后继续往下看
postProcessBeanFactory(beanFactory);方法是提供给子类的扩展点，到这里的时候，所有的 Bean 都加载、注册完成了，但是都还没有初始化，具体的子类可以在这步的时候添加一些特殊的 BeanFactoryPostProcessor 的实现类，来完成一些其他的操作。
接下来是invokeBeanFactoryPostProcessors(beanFactory);这个方法是调用 BeanFactoryPostProcessor 各个实现类的 postProcessBeanFactory(factory) 方法；
然后是registerBeanPostProcessors(beanFactory);这个方法注册 BeanPostProcessor 的实现类，和上面的BeanFactoryPostProcessor 是有区别的，这个方法调用的其实是PostProcessorRegistrationDelegate类的registerBeanPostProcessors方法；这个类里面有个内部类BeanPostProcessorChecker，BeanPostProcessorChecker里面有两个方法postProcessBeforeInitialization和postProcessAfterInitialization，这两个方法分别在 Bean 初始化之前和初始化之后得到执行。然后回到refresh()方法中继续往下看
initMessageSource();方法是初始化当前 ApplicationContext 的 MessageSource，国际化处理，继续往下
initApplicationEventMulticaster();方法初始化当前 ApplicationContext 的事件广播器继续往下
onRefresh();方法初始化一些特殊的 Bean（在初始化 singleton beans 之前）；继续往下
registerListeners();方法注册事件监听器，监听器需要实现 ApplicationListener 接口；继续往下
重点到了：finishBeanFactoryInitialization(beanFactory);初始化所有的 singleton beans（单例bean），懒加载（non-lazy-init）的除外，这个方法也是等会细说
finishRefresh();方法是最后一步，广播事件，ApplicationContext 初始化完成
这就是整个refresh()方法调用的所有方法。这里只是简单描述一下，我们重点来看ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();和finishBeanFactoryInitialization(beanFactory);这两个方法。

先说ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); 这一步上面简单介绍过了，作用是把配置文件解析成一个个Bean，并且注册到BeanFactory中，点进去源码：

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
	refreshBeanFactory();
	ConfigurableListableBeanFactory beanFactory = getBeanFactory();
	if (logger.isDebugEnabled()) {
		logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
	}
	return beanFactory;
}

这个方法中第一步refreshBeanFactory();方法的作用是关闭旧的 BeanFactory (如果有)，创建新的 BeanFactory，加载 Bean 定义、注册 Bean 等，然后getBeanFactory();就是返回刚刚创建的 BeanFactory，我们进入refreshBeanFactory();方法，在AbstractRefreshableApplicationContext类中：

@Override
protected final void refreshBeanFactory() throws BeansException {
	if (hasBeanFactory()) {
		destroyBeans();
		closeBeanFactory();
	}
	try {
		DefaultListableBeanFactory beanFactory = createBeanFactory();
		beanFactory.setSerializationId(getId());
		customizeBeanFactory(beanFactory);
		loadBeanDefinitions(beanFactory);
		synchronized (this.beanFactoryMonitor) {
			this.beanFactory = beanFactory;
		}
	}
	catch (IOException ex) {
		throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
	}
}

这个refreshBeanFactory()方法首先如果 ApplicationContext 中已经加载过 BeanFactory
了，销毁所有 Bean，关闭 BeanFactory；这里指的是当前ApplicationContext 是否有 BeanFactory。
然后createBeanFactory();初始化一个DefaultListableBeanFactory，这个DefaultListableBeanFactory是很重的一个类，为什么重要呢？可以看文章开头的BeanFactory继承图，DefaultListableBeanFactory是位于最下面的，他往上能走完BeanFactory继承图所有，所以他可以说是功能最大的BeanFactory。
beanFactory.setSerializationId(getId());方法用于 BeanFactory 的序列化
customizeBeanFactory(beanFactory);方法设置 BeanFactory 的两个配置属性：是否允许 Bean 覆盖、是否允许循环引用，这个等会细说
loadBeanDefinitions(beanFactory);这个方法很重要：加载 Bean 到 BeanFactory 中，也是等会细说
下面看customizeBeanFactory(beanFactory);方法，这个方法作用：是否允许 Bean 覆盖、是否允许循环引用，这是什么意思呢？这就要说到BeanDefinition了，这里的 BeanDefinition 就是我们所说的 Spring 的 Bean，我们自己定义的各个 Bean 其实会转换成一个个 BeanDefinition 存在于 Spring 的 BeanFactory 中，下面贴出来开头我说的大神的关于BeanDefinition接口的代码注释：

public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {

   // 我们可以看到，默认只提供 sington 和 prototype 两种，
   // 很多读者可能知道还有 request, session, globalSession, application, websocket 这几种，
   // 不过，它们属于基于 web 的扩展。
   String SCOPE_SINGLETON = ConfigurableBeanFactory.SCOPE_SINGLETON;
   String SCOPE_PROTOTYPE = ConfigurableBeanFactory.SCOPE_PROTOTYPE;

   // 比较不重要，直接跳过吧
   int ROLE_APPLICATION = 0;
   int ROLE_SUPPORT = 1;
   int ROLE_INFRASTRUCTURE = 2;

   // 设置父 Bean，这里涉及到 bean 继承，不是 java 继承。请参见附录的详细介绍
   // 一句话就是：继承父 Bean 的配置信息而已
   void setParentName(String parentName);

   // 获取父 Bean
   String getParentName();

   // 设置 Bean 的类名称，将来是要通过反射来生成实例的
   void setBeanClassName(String beanClassName);

   // 获取 Bean 的类名称
   String getBeanClassName();


   // 设置 bean 的 scope
   void setScope(String scope);

   String getScope();

   // 设置是否懒加载
   void setLazyInit(boolean lazyInit);

   boolean isLazyInit();

   // 设置该 Bean 依赖的所有的 Bean，注意，这里的依赖不是指属性依赖(如 @Autowire 标记的)，
   // 是 depends-on="" 属性设置的值。
   void setDependsOn(String... dependsOn);

   // 返回该 Bean 的所有依赖
   String[] getDependsOn();

   // 设置该 Bean 是否可以注入到其他 Bean 中，只对根据类型注入有效，
   // 如果根据名称注入，即使这边设置了 false，也是可以的
   void setAutowireCandidate(boolean autowireCandidate);

   // 该 Bean 是否可以注入到其他 Bean 中
   boolean isAutowireCandidate();

   // 主要的。同一接口的多个实现，如果不指定名字的话，Spring 会优先选择设置 primary 为 true 的 bean
   void setPrimary(boolean primary);

   // 是否是 primary 的
   boolean isPrimary();

   // 如果该 Bean 采用工厂方法生成，指定工厂名称。对工厂不熟悉的读者，请参加附录
   // 一句话就是：有些实例不是用反射生成的，而是用工厂模式生成的
   void setFactoryBeanName(String factoryBeanName);
   // 获取工厂名称
   String getFactoryBeanName();
   // 指定工厂类中的 工厂方法名称
   void setFactoryMethodName(String factoryMethodName);
   // 获取工厂类中的 工厂方法名称
   String getFactoryMethodName();

   // 构造器参数
   ConstructorArgumentValues getConstructorArgumentValues();

   // Bean 中的属性值，后面给 bean 注入属性值的时候会说到
   MutablePropertyValues getPropertyValues();

   // 是否 singleton
   boolean isSingleton();

   // 是否 prototype
   boolean isPrototype();

   // 如果这个 Bean 是被设置为 abstract，那么不能实例化，
   // 常用于作为 父bean 用于继承，其实也很少用......
   boolean isAbstract();

   int getRole();
   String getDescription();
   String getResourceDescription();
   BeanDefinition getOriginatingBeanDefinition();
}

BeanDefinition 的覆盖问题就是在配置文件中定义 bean 时使用了相同的 id 或 name，默认情况下，allowBeanDefinitionOverriding 属性为 null，如果在同一配置文件中重复了，会抛错，但是如果不是同一配置文件中，会发生覆盖。
循环引用：A 依赖 B，而 B 依赖 A。或 A 依赖 B，B 依赖 C，而 C 依赖 A。
默认情况下，Spring 允许循环依赖，当然如果你在 A 的构造方法中依赖 B，在 B 的构造方法中依赖 A 是不行的。

再看loadBeanDefinitions(beanFactory) 方法，这个方法将根据配置，加载各个 Bean，然后放到 BeanFactory 中。先贴上源码：AbstractXmlApplicationContext类中

@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
	// Create a new XmlBeanDefinitionReader for the given BeanFactory.
	XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

	// Configure the bean definition reader with this context's
	// resource loading environment.
	beanDefinitionReader.setEnvironment(this.getEnvironment());
	beanDefinitionReader.setResourceLoader(this);
	beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

	// Allow a subclass to provide custom initialization of the reader,
	// then proceed with actually loading the bean definitions.
	initBeanDefinitionReader(beanDefinitionReader);
	loadBeanDefinitions(beanDefinitionReader);
}

这个源码中我们重点看loadBeanDefinitions(beanDefinitionReader);，再点进去源码：

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
	Resource[] configResources = getConfigResources();
	if (configResources != null) {
		reader.loadBeanDefinitions(configResources);
	}
	String[] configLocations = getConfigLocations();
	if (configLocations != null) {
		reader.loadBeanDefinitions(configLocations);
	}
}

在这段源码中我么可以看到2个reader.loadBeanDefinitions()方法，其实两个最终都到了：
AbstractBeanDefinitionReader类中：

@Override
public int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException {
	Assert.notNull(resources, "Resource array must not be null");
	int counter = 0;
	for (Resource resource : resources) {
		counter += loadBeanDefinitions(resource);
	}
	return counter;
}

这个方法里面，for循环每一个文件是一个resource，最终返回 counter，表示总共加载了多少的 BeanDefinition。
进入loadBeanDefinitions(resource);方法，看源码：
XmlBeanDefinitionReader类中的：

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
	Assert.notNull(encodedResource, "EncodedResource must not be null");
	if (logger.isInfoEnabled()) {
		logger.info("Loading XML bean definitions from " + encodedResource);
	}

	Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
	if (currentResources == null) {
		currentResources = new HashSet<>(4);
		this.resourcesCurrentlyBeingLoaded.set(currentResources);
	}
	if (!currentResources.add(encodedResource)) {
		throw new BeanDefinitionStoreException(
				"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
	}
	try {
		InputStream inputStream = encodedResource.getResource().getInputStream();
		try {
			InputSource inputSource = new InputSource(inputStream);
			if (encodedResource.getEncoding() != null) {
				inputSource.setEncoding(encodedResource.getEncoding());
			}
			return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
		}
		finally {
			inputStream.close();
		}
	}
	catch (IOException ex) {
		throw new BeanDefinitionStoreException(
				"IOException parsing XML document from " + encodedResource.getResource(), ex);
	}
	finally {
		currentResources.remove(encodedResource);
		if (currentResources.isEmpty()) {
			this.resourcesCurrentlyBeingLoaded.remove();
		}
	}
}

这个源码的核心在doLoadBeanDefinitions(inputSource, encodedResource.getResource());继续进源码：

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
		throws BeanDefinitionStoreException {
	try {
		Document doc = doLoadDocument(inputSource, resource);
		return registerBeanDefinitions(doc, resource);
	}
	catch (BeanDefinitionStoreException ex) {
		throw ex;
	}
	catch (SAXParseException ex) {
		throw new XmlBeanDefinitionStoreException(resource.getDescription(),
				"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
	}
	catch (SAXException ex) {
		throw new XmlBeanDefinitionStoreException(resource.getDescription(),
				"XML document from " + resource + " is invalid", ex);
	}
	catch (ParserConfigurationException ex) {
		throw new BeanDefinitionStoreException(resource.getDescription(),
				"Parser configuration exception parsing XML from " + resource, ex);
	}
	catch (IOException ex) {
		throw new BeanDefinitionStoreException(resource.getDescription(),
				"IOException parsing XML document from " + resource, ex);
	}
	catch (Throwable ex) {
		throw new BeanDefinitionStoreException(resource.getDescription(),
				"Unexpected exception parsing XML document from " + resource, ex);
	}
}

在这个方法里面可以看到doLoadDocument(inputSource, resource);方法是将 xml 文件转换为 Document 对象，然后继续进源码：

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
	BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
	int countBefore = getRegistry().getBeanDefinitionCount();
	documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
	return getRegistry().getBeanDefinitionCount() - countBefore;
}

这个方法里面看：documentReader.registerBeanDefinitions(doc, createReaderContext(resource));点进去：

@Override
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
	this.readerContext = readerContext;
	logger.debug("Loading bean definitions");
	Element root = doc.getDocumentElement();
	doRegisterBeanDefinitions(root);
}

里面的doRegisterBeanDefinitions(root);方法就是从 xml 根节点开始解析文件，经过很多的步骤，一个配置文件终于转换为一颗 DOM 树了，注意，这里指的是其中一个配置文件，不是所有的，可以看到上面有个 for 循环的(loadBeanDefinitions(resource);是在for循环里面的)，进入doRegisterBeanDefinitions(root);方法，

protected void doRegisterBeanDefinitions(Element root) {
	// Any nested <beans> elements will cause recursion in this method. In
	// order to propagate and preserve <beans> default-* attributes correctly,
	// keep track of the current (parent) delegate, which may be null. Create
	// the new (child) delegate with a reference to the parent for fallback purposes,
	// then ultimately reset this.delegate back to its original (parent) reference.
	// this behavior emulates a stack of delegates without actually necessitating one.
	BeanDefinitionParserDelegate parent = this.delegate;
	this.delegate = createDelegate(getReaderContext(), root, parent);

	if (this.delegate.isDefaultNamespace(root)) {
		String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
		if (StringUtils.hasText(profileSpec)) {
			String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
					profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
			if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
				if (logger.isInfoEnabled()) {
					logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
							"] not matching: " + getReaderContext().getResource());
				}
				return;
			}
		}
	}

	preProcessXml(root);
	parseBeanDefinitions(root, this.delegate);
	postProcessXml(root);

	this.delegate = parent;
}

在这里开始就是真正的解析xml parseBeanDefinitions(root, this.delegate);这个方法往里面，具体你们点进去看吧，实在太多了，或者你们可以去看文章开头的大神的博客，这里就不贴出来了。代码到这里之后呢，Bean容器就已经算是初始化完成了。然后无偶们回到refresh() 方法，就是文章前部分的那个refresh() 方法…TvT
再贴一下refresh() 方法的源码吧，方便阅读：
我直接贴大神带注释的代码吧，看起来更方便一点

@Override
public void refresh() throws BeansException, IllegalStateException {
   // 来个锁，不然 refresh() 还没结束，你又来个启动或销毁容器的操作，那不就乱套了嘛
   synchronized (this.startupShutdownMonitor) {

      // 准备工作，记录下容器的启动时间、标记“已启动”状态、处理配置文件中的占位符
      prepareRefresh();

      // 这步比较关键，这步完成后，配置文件就会解析成一个个 Bean 定义，注册到 BeanFactory 中，
      // 当然，这里说的 Bean 还没有初始化，只是配置信息都提取出来了，
      // 注册也只是将这些信息都保存到了注册中心(说到底核心是一个 beanName-> beanDefinition 的 map)
      ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

      // 设置 BeanFactory 的类加载器，添加几个 BeanPostProcessor，手动注册几个特殊的 bean
      // 这块待会会展开说
      prepareBeanFactory(beanFactory);

      try {
         // 【这里需要知道 BeanFactoryPostProcessor 这个知识点，Bean 如果实现了此接口，
         // 那么在容器初始化以后，Spring 会负责调用里面的 postProcessBeanFactory 方法。】

         // 这里是提供给子类的扩展点，到这里的时候，所有的 Bean 都加载、注册完成了，但是都还没有初始化
         // 具体的子类可以在这步的时候添加一些特殊的 BeanFactoryPostProcessor 的实现类或做点什么事
         postProcessBeanFactory(beanFactory);
         // 调用 BeanFactoryPostProcessor 各个实现类的 postProcessBeanFactory(factory) 回调方法
         invokeBeanFactoryPostProcessors(beanFactory);          



         // 注册 BeanPostProcessor 的实现类，注意看和 BeanFactoryPostProcessor 的区别
         // 此接口两个方法: postProcessBeforeInitialization 和 postProcessAfterInitialization
         // 两个方法分别在 Bean 初始化之前和初始化之后得到执行。这里仅仅是注册，之后会看到回调这两方法的时机
         registerBeanPostProcessors(beanFactory);

         // 初始化当前 ApplicationContext 的 MessageSource，国际化这里就不展开说了，不然没完没了了
         initMessageSource();

         // 初始化当前 ApplicationContext 的事件广播器，这里也不展开了
         initApplicationEventMulticaster();

         // 从方法名就可以知道，典型的模板方法(钩子方法)，不展开说
         // 具体的子类可以在这里初始化一些特殊的 Bean（在初始化 singleton beans 之前）
         onRefresh();

         // 注册事件监听器，监听器需要实现 ApplicationListener 接口。这也不是我们的重点，过
         registerListeners();

         // 重点，重点，重点
         // 初始化所有的 singleton beans
         //（lazy-init 的除外）
         finishBeanFactoryInitialization(beanFactory);

         // 最后，广播事件，ApplicationContext 初始化完成，不展开
         finishRefresh();
      }

      catch (BeansException ex) {
         if (logger.isWarnEnabled()) {
            logger.warn("Exception encountered during context initialization - " +
                  "cancelling refresh attempt: " + ex);
         }

         // Destroy already created singletons to avoid dangling resources.
         // 销毁已经初始化的 singleton 的 Beans，以免有些 bean 会一直占用资源
         destroyBeans();

         // Reset 'active' flag.
         cancelRefresh(ex);

         // 把异常往外抛
         throw ex;
      }

      finally {
         // Reset common introspection caches in Spring's core, since we
         // might not ever need metadata for singleton beans anymore...
         resetCommonCaches();
      }
   }
}

这个注释相当清楚，我们直接来看finishBeanFactoryInitialization(beanFactory);方法吧，到这一步为止BeanFactory 已经创建完成，并且所有的实现了 BeanFactoryPostProcessor 接口的 Bean 都已经初始化并且其中的 postProcessBeanFactory(factory) 方法已经得到回调执行了。而且 Spring 已经“手动”注册了一些特殊的 Bean，如 ‘environment’、‘systemProperties’ 等。剩下的就是初始化 singleton beans 了，我们知道它们是单例的，如果没有设置懒加载，那么 Spring 会在接下来初始化所有的 singleton beans。点进去看源码：

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
	// Initialize conversion service for this context.
	if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
			beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
		beanFactory.setConversionService(
				beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
	}

	// Register a default embedded value resolver if no bean post-processor
	// (such as a PropertyPlaceholderConfigurer bean) registered any before:
	// at this point, primarily for resolution in annotation attribute values.
	if (!beanFactory.hasEmbeddedValueResolver()) {
		beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
	}

	// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
	String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
	for (String weaverAwareName : weaverAwareNames) {
		getBean(weaverAwareName);
	}

	// Stop using the temporary ClassLoader for type matching.
	beanFactory.setTempClassLoader(null);

	// Allow for caching all bean definition metadata, not expecting further changes.
	beanFactory.freezeConfiguration();

	// Instantiate all remaining (non-lazy-init) singletons.
	beanFactory.preInstantiateSingletons();
}

在这段代码中：首先初始化名字为 “conversionService” 的 Bean，为什么是conversionService 呢？原因是注册这个bean之后，类似于前端传给后端的非基础类型和基础类型的包装类之外，其他的就可以考虑采用ConversionService来进行类型等的转换，初始化这个 “conversionService” 实在上面源码中的beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));进行的。然后直接跳过来到beanFactory.preInstantiateSingletons();这个方法，这里开始初始化。点进去：
DefaultListableBeanFactory这个类中：

@Override
public void preInstantiateSingletons() throws BeansException {
	if (logger.isDebugEnabled()) {
		logger.debug("Pre-instantiating singletons in " + this);
	}

	// Iterate over a copy to allow for init methods which in turn register new bean definitions.
	// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
	List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

	// Trigger initialization of all non-lazy singleton beans...
	for (String beanName : beanNames) {
		RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
		if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
			if (isFactoryBean(beanName)) {
				Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
				if (bean instanceof FactoryBean) {
					final FactoryBean<?> factory = (FactoryBean<?>) bean;
					boolean isEagerInit;
					if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
						isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
										((SmartFactoryBean<?>) factory)::isEagerInit,
								getAccessControlContext());
					}
					else {
						isEagerInit = (factory instanceof SmartFactoryBean &&
								((SmartFactoryBean<?>) factory).isEagerInit());
					}
					if (isEagerInit) {
						getBean(beanName);
					}
				}
			}
			else {
				getBean(beanName);
			}
		}
	}

	// Trigger post-initialization callback for all applicable beans...
	for (String beanName : beanNames) {
		Object singletonInstance = getSingleton(beanName);
		if (singletonInstance instanceof SmartInitializingSingleton) {
			final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
			if (System.getSecurityManager() != null) {
				AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
					smartSingleton.afterSingletonsInstantiated();
					return null;
				}, getAccessControlContext());
			}
			else {
				smartSingleton.afterSingletonsInstantiated();
			}
		}
	}
}

在这段源码中，this.beanDefinitionNames 保存了所有的 beanNames，然后再循环，判断非抽象、非懒加载的 singletons，如果是FactoryBean的话，在 beanName 前面加上 ‘&’ 符号，再调用getBean(beanName);，如果是普通的bean，那么直接getBean(beanName);，这里都是在循环中的，循环结束后，所有的非单例bean就初始化完成了，接着下面如果我们定义的 bean 是实现了 SmartInitializingSingleton 接口的，那么在smartSingleton.afterSingletonsInstantiated();这里得到回调，我们直接进入getBean()方法：
可以看到最终到了AbstractBeanFactory类的doGetBean方法：

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
			@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

	final String beanName = transformedBeanName(name);
	Object bean;

	// Eagerly check singleton cache for manually registered singletons.
	Object sharedInstance = getSingleton(beanName);
	if (sharedInstance != null && args == null) {
		if (logger.isDebugEnabled()) {
			if (isSingletonCurrentlyInCreation(beanName)) {
				logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
						"' that is not fully initialized yet - a consequence of a circular reference");
			}
			else {
				logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
			}
		}
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}

	else {
		// Fail if we're already creating this bean instance:
		// We're assumably within a circular reference.
		if (isPrototypeCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}

		// Check if bean definition exists in this factory.
		BeanFactory parentBeanFactory = getParentBeanFactory();
		if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
			// Not found -> check parent.
			String nameToLookup = originalBeanName(name);
			if (parentBeanFactory instanceof AbstractBeanFactory) {
				return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
						nameToLookup, requiredType, args, typeCheckOnly);
			}
			else if (args != null) {
				// Delegation to parent with explicit args.
				return (T) parentBeanFactory.getBean(nameToLookup, args);
			}
			else {
				// No args -> delegate to standard getBean method.
				return parentBeanFactory.getBean(nameToLookup, requiredType);
			}
		}

		if (!typeCheckOnly) {
			markBeanAsCreated(beanName);
		}

		try {
			final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
			checkMergedBeanDefinition(mbd, beanName, args);

			// Guarantee initialization of beans that the current bean depends on.
			String[] dependsOn = mbd.getDependsOn();
			if (dependsOn != null) {
				for (String dep : dependsOn) {
					if (isDependent(beanName, dep)) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
					}
					registerDependentBean(dep, beanName);
					try {
						getBean(dep);
					}
					catch (NoSuchBeanDefinitionException ex) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
					}
				}
			}

			// Create bean instance.
			if (mbd.isSingleton()) {
				sharedInstance = getSingleton(beanName, () -> {
					try {
						return createBean(beanName, mbd, args);
					}
					catch (BeansException ex) {
						// Explicitly remove instance from singleton cache: It might have been put there
						// eagerly by the creation process, to allow for circular reference resolution.
						// Also remove any beans that received a temporary reference to the bean.
						destroySingleton(beanName);
						throw ex;
					}
				});
				bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
			}

			else if (mbd.isPrototype()) {
				// It's a prototype -> create a new instance.
				Object prototypeInstance = null;
				try {
					beforePrototypeCreation(beanName);
					prototypeInstance = createBean(beanName, mbd, args);
				}
				finally {
					afterPrototypeCreation(beanName);
				}
				bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
			}

			else {
				String scopeName = mbd.getScope();
				final Scope scope = this.scopes.get(scopeName);
				if (scope == null) {
					throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
				}
				try {
					Object scopedInstance = scope.get(beanName, () -> {
						beforePrototypeCreation(beanName);
						try {
							return createBean(beanName, mbd, args);
						}
						finally {
							afterPrototypeCreation(beanName);
						}
					});
					bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
				}
				catch (IllegalStateException ex) {
					throw new BeanCreationException(beanName,
							"Scope '" + scopeName + "' is not active for the current thread; consider " +
							"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
							ex);
				}
			}
		}
		catch (BeansException ex) {
			cleanupAfterBeanCreationFailure(beanName);
			throw ex;
		}
	}

	// Check if required type matches the type of the actual bean instance.
	if (requiredType != null && !requiredType.isInstance(bean)) {
		try {
			T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
			if (convertedBean == null) {
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
			return convertedBean;
		}
		catch (TypeMismatchException ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Failed to convert bean '" + name + "' to required type '" +
						ClassUtils.getQualifiedName(requiredType) + "'", ex);
			}
			throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
		}
	}
	return (T) bean;
}

在这段源码中：

final String beanName = transformedBeanName(name);获取一个正确 beanName，处理两种情况，一个是前面说的 FactoryBean(前面带 ‘&’)，一个是别名问题，因为这个方法是 getBean，获取 Bean 用的。
Object bean 这个是返回值
Object sharedInstance = getSingleton(beanName);检查是否已经创建过
if (sharedInstance != null && args == null) 这个判断是否已经创建过，还有args 传参其实是 null 的，但是如果 args 不为空的时候，那么意味着调用方不是希望获取 Bean，而是创建 Bean，在这个if中，bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);如果是普通 Bean 的话，直接返回 sharedInstance，如果是 FactoryBean 的话，返回它创建的那个实例对象。然后一系列检查
看到final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);这里之前，检查完成，开始准备创建Bean 了，对于 singleton 的 Bean 来说，容器中还没创建过此 Bean；对于 prototype 的 Bean 来说，本来就是要创建一个新的 Bean。
String[] dependsOn = mbd.getDependsOn();先初始化依赖的所有 Bean，注意，这里的依赖指的是 depends-on 中定义的依赖
registerDependentBean(dep, beanName); 注册依赖关系；然后getBean(dep); 先初始化被依赖项
这里if (mbd.isSingleton()) 如果是单例那么表达式->createBean(beanName, mbd, args);执行创建 Bean
这里else if (mbd.isPrototype())如果是 prototype scope 的，创建 prototype 的实例，prototypeInstance = createBean(beanName, mbd, args);执行创建 Bean
这里else如果不是 singleton 和 prototype 的话，需要委托给相应的实现类来处理，createBean(beanName, mbd, args);
if (requiredType != null && !requiredType.isInstance(bean)) 最后检查一下类型是否正确，不正确抛异常，正确流返回创建的bean
然后在进入createBean(beanName, mbd, args); …TvT
AbstractAutowireCapableBeanFactory类中：

@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
		throws BeanCreationException {

	if (logger.isDebugEnabled()) {
		logger.debug("Creating instance of bean '" + beanName + "'");
	}
	RootBeanDefinition mbdToUse = mbd;

	// Make sure bean class is actually resolved at this point, and
	// clone the bean definition in case of a dynamically resolved Class
	// which cannot be stored in the shared merged bean definition.
	Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
	if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
		mbdToUse = new RootBeanDefinition(mbd);
		mbdToUse.setBeanClass(resolvedClass);
	}

	// Prepare method overrides.
	try {
		mbdToUse.prepareMethodOverrides();
	}
	catch (BeanDefinitionValidationException ex) {
		throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
				beanName, "Validation of method overrides failed", ex);
	}

	try {
		// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
		Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
		if (bean != null) {
			return bean;
		}
	}
	catch (Throwable ex) {
		throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
				"BeanPostProcessor before instantiation of bean failed", ex);
	}

	try {
		Object beanInstance = doCreateBean(beanName, mbdToUse, args);
		if (logger.isDebugEnabled()) {
			logger.debug("Finished creating instance of bean '" + beanName + "'");
		}
		return beanInstance;
	}
	catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
		// A previously detected exception with proper bean creation context already,
		// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
		throw ex;
	}
	catch (Throwable ex) {
		throw new BeanCreationException(
				mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
	}
}

这里来到AbstractAutowireCapableBeanFactory类主要是为了采用 @Autowired 注解注入属性值(这个很常用，例子很多啊，就普通的依赖注入注解)
然后看到Class<?> resolvedClass = resolveBeanClass(mbd, beanName);确保 BeanDefinition 中的 Class 被加载；
往下Object bean = resolveBeforeInstantiation(beanName, mbdToUse);让 InstantiationAwareBeanPostProcessor 在这一步有机会返回代理，和AOP有关；继续往下Object beanInstance = doCreateBean(beanName, mbdToUse, args);这是重点，当然是点进去啦：

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
			throws BeanCreationException {

	// Instantiate the bean.
	BeanWrapper instanceWrapper = null;
	if (mbd.isSingleton()) {
		instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
	}
	if (instanceWrapper == null) {
		instanceWrapper = createBeanInstance(beanName, mbd, args);
	}
	final Object bean = instanceWrapper.getWrappedInstance();
	Class<?> beanType = instanceWrapper.getWrappedClass();
	if (beanType != NullBean.class) {
		mbd.resolvedTargetType = beanType;
	}

	// Allow post-processors to modify the merged bean definition.
	synchronized (mbd.postProcessingLock) {
		if (!mbd.postProcessed) {
			try {
				applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
			}
			catch (Throwable ex) {
				throw new BeanCreationException(mbd.getResourceDescription(), beanName,
						"Post-processing of merged bean definition failed", ex);
			}
			mbd.postProcessed = true;
		}
	}

	// Eagerly cache singletons to be able to resolve circular references
	// even when triggered by lifecycle interfaces like BeanFactoryAware.
	boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
			isSingletonCurrentlyInCreation(beanName));
	if (earlySingletonExposure) {
		if (logger.isDebugEnabled()) {
			logger.debug("Eagerly caching bean '" + beanName +
					"' to allow for resolving potential circular references");
		}
		addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
	}

	// Initialize the bean instance.
	Object exposedObject = bean;
	try {
		populateBean(beanName, mbd, instanceWrapper);
		exposedObject = initializeBean(beanName, exposedObject, mbd);
	}
	catch (Throwable ex) {
		if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
			throw (BeanCreationException) ex;
		}
		else {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
		}
	}

	if (earlySingletonExposure) {
		Object earlySingletonReference = getSingleton(beanName, false);
		if (earlySingletonReference != null) {
			if (exposedObject == bean) {
				exposedObject = earlySingletonReference;
			}
			else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
				String[] dependentBeans = getDependentBeans(beanName);
				Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
				for (String dependentBean : dependentBeans) {
					if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
						actualDependentBeans.add(dependentBean);
					}
				}
				if (!actualDependentBeans.isEmpty()) {
					throw new BeanCurrentlyInCreationException(beanName,
							"Bean with name '" + beanName + "' has been injected into other beans [" +
							StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
							"] in its raw version as part of a circular reference, but has eventually been " +
							"wrapped. This means that said other beans do not use the final version of the " +
							"bean. This is often the result of over-eager type matching - consider using " +
							"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
				}
			}
		}
	}

	// Register bean as disposable.
	try {
		registerDisposableBeanIfNecessary(beanName, bean, mbd);
	}
	catch (BeanDefinitionValidationException ex) {
		throw new BeanCreationException(
				mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
	}

	return exposedObject;
}

if (instanceWrapper == null)如果进入这个if，说明不是 FactoryBean，instanceWrapper = createBeanInstance(beanName, mbd, args);这里实例化 Bean；下面再说
然后做一些处理比如循环依赖等，直到看到populateBean(beanName, mbd, instanceWrapper);这个代码，这一步负责属性装配，很重要，因为前面的实例只是实例化了，并没有设值，这里就是设值
然后看createBeanInstance(beanName, mbd, args);方法，实例化bean 的

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
	// Make sure bean class is actually resolved at this point.
	Class<?> beanClass = resolveBeanClass(mbd, beanName);

	if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
		throw new BeanCreationException(mbd.getResourceDescription(), beanName,
				"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
	}

	Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
	if (instanceSupplier != null) {
		return obtainFromSupplier(instanceSupplier, beanName);
	}

	if (mbd.getFactoryMethodName() != null)  {
		return instantiateUsingFactoryMethod(beanName, mbd, args);
	}

	// Shortcut when re-creating the same bean...
	boolean resolved = false;
	boolean autowireNecessary = false;
	if (args == null) {
		synchronized (mbd.constructorArgumentLock) {
			if (mbd.resolvedConstructorOrFactoryMethod != null) {
				resolved = true;
				autowireNecessary = mbd.constructorArgumentsResolved;
			}
		}
	}
	if (resolved) {
		if (autowireNecessary) {
			return autowireConstructor(beanName, mbd, null, null);
		}
		else {
			return instantiateBean(beanName, mbd);
		}
	}

	// Candidate constructors for autowiring?
	Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
	if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
			mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
		return autowireConstructor(beanName, mbd, ctors, args);
	}

	// No special handling: simply use no-arg constructor.
	return instantiateBean(beanName, mbd);
}

Class<?> beanClass = resolveBeanClass(mbd, beanName);首先确保已经加载过这个class
然后if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed())检验一下权限
if (instanceSupplier != null) 如果存在 Supplier 回调，则调用 obtainFromSupplier() 进行初始化
if (mbd.getFactoryMethodName() != null) 采用工厂方法实例化
if (resolved) { if (autowireNecessary) 如果到这里的话是构造函数依赖注入autowireConstructor(beanName, mbd, null, null);
else如果没进上面的if (resolved) { if (autowireNecessary)这个if，那么就是无参构造函数 instantiateBean(beanName, mbd);
determineConstructorsFromBeanPostProcessors(beanClass, beanName); 这个和后面的if判断作用是：判断是否采用有参构造函数，如果是那么就采用构造函数依赖注入autowireConstructor(beanName, mbd, ctors, args);
这里我们选无参构造点进去看一下：

protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
	try {
		Object beanInstance;
		final BeanFactory parent = this;
		if (System.getSecurityManager() != null) {
			beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->
					getInstantiationStrategy().instantiate(mbd, beanName, parent),
					getAccessControlContext());
		}
		else {
			beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
		}
		BeanWrapper bw = new BeanWrapperImpl(beanInstance);
		initBeanWrapper(bw);
		return bw;
	}
	catch (Throwable ex) {
		throw new BeanCreationException(
				mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
	}
}

在上面的无参构造源码里面，真正的实例化是getInstantiationStrategy().instantiate(mbd, beanName, parent);这段代码，这行执行完成之后，包装成BeanWrapper就直接返回了，我们再点进去instantiate方法看看，在SimpleInstantiationStrategy类中:

@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
	// Don't override the class with CGLIB if no overrides.
	if (!bd.hasMethodOverrides()) {
		Constructor<?> constructorToUse;
		synchronized (bd.constructorArgumentLock) {
			constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
			if (constructorToUse == null) {
				final Class<?> clazz = bd.getBeanClass();
				if (clazz.isInterface()) {
					throw new BeanInstantiationException(clazz, "Specified class is an interface");
				}
				try {
					if (System.getSecurityManager() != null) {
						constructorToUse = AccessController.doPrivileged(
								(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
					}
					else {
						constructorToUse =	clazz.getDeclaredConstructor();
					}
					bd.resolvedConstructorOrFactoryMethod = constructorToUse;
				}
				catch (Throwable ex) {
					throw new BeanInstantiationException(clazz, "No default constructor found", ex);
				}
			}
		}
		return BeanUtils.instantiateClass(constructorToUse);
	}
	else {
		// Must generate CGLIB subclass.
		return instantiateWithMethodInjection(bd, beanName, owner);
	}
}

在这个方法里面，如果if (!bd.hasMethodOverrides())进入这个if里面，那说明不存在方法覆写，使用 java 反射进行实例化，如果不进入使用 CGLIB进行实例化
下面这个就是反射实例化

BeanUtils.instantiateClass(constructorToUse);

另外存在方法覆写就是下面这个CGLIB进行实例化

instantiateWithMethodInjection(bd, beanName, owner);

到这里。终于bean被实例化了，然后再看看属性怎么注入的。。。TvT
回到AbstractAutowireCapableBeanFactory类的doCreateBean方法，我们刚才说到createBeanInstance是实例化，接下来就是属性注入方法：populateBean(beanName, mbd, instanceWrapper);点进去看：

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
	if (bw == null) {
		if (mbd.hasPropertyValues()) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
		}
		else {
			// Skip property population phase for null instance.
			return;
		}
	}

	// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
	// state of the bean before properties are set. This can be used, for example,
	// to support styles of field injection.
	boolean continueWithPropertyPopulation = true;

	if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
		for (BeanPostProcessor bp : getBeanPostProcessors()) {
			if (bp instanceof InstantiationAwareBeanPostProcessor) {
				InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
				if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
					continueWithPropertyPopulation = false;
					break;
				}
			}
		}
	}

	if (!continueWithPropertyPopulation) {
		return;
	}

	PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);

	if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
		MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
		// Add property values based on autowire by name if applicable.
		if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {
			autowireByName(beanName, mbd, bw, newPvs);
		}
		// Add property values based on autowire by type if applicable.
		if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
			autowireByType(beanName, mbd, bw, newPvs);
		}
		pvs = newPvs;
	}

	boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
	boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);

	if (hasInstAwareBpps || needsDepCheck) {
		if (pvs == null) {
			pvs = mbd.getPropertyValues();
		}
		PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
		if (hasInstAwareBpps) {
			for (BeanPostProcessor bp : getBeanPostProcessors()) {
				if (bp instanceof InstantiationAwareBeanPostProcessor) {
					InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
					pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
					if (pvs == null) {
						return;
					}
				}
			}
		}
		if (needsDepCheck) {
			checkDependencies(beanName, mbd, filteredPds, pvs);
		}
	}

	if (pvs != null) {
		applyPropertyValues(beanName, mbd, bw, pvs);
	}
}

这这段源码中，能到这里说明我们的bean已经实例化完成了(工厂方法或者构造方法)，开始设置属性值，

在这段源码中往下看，找到if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME)这里表示通过名字找到所有属性值，如果是 bean 依赖，先初始化依赖的 bean。记录依赖关系。
还可以看到if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE)通过类型装配
再往下pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);对采用 @Autowired、@Value 注解的依赖进行设值
最后面applyPropertyValues(beanName, mbd, bw, pvs);就是设置 bean 实例的属性值了
到这里，属性注入就完成了，然后回到AbstractAutowireCapableBeanFactory类的doCreateBean方法：populateBean(beanName, mbd, instanceWrapper);执行完成之后，调用的exposedObject = initializeBean(beanName, exposedObject, mbd);，这个方法的作用是处理各种回调。点进去看看吧：

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
	if (System.getSecurityManager() != null) {
		AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
			invokeAwareMethods(beanName, bean);
			return null;
		}, getAccessControlContext());
	}
	else {
		invokeAwareMethods(beanName, bean);
	}

	Object wrappedBean = bean;
	if (mbd == null || !mbd.isSynthetic()) {
		wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
	}

	try {
		invokeInitMethods(beanName, wrappedBean, mbd);
	}
	catch (Throwable ex) {
		throw new BeanCreationException(
				(mbd != null ? mbd.getResourceDescription() : null),
				beanName, "Invocation of init method failed", ex);
	}
	if (mbd == null || !mbd.isSynthetic()) {
		wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
	}

	return wrappedBean;
}

这段代码中：

invokeAwareMethods(beanName, bean);表示如果 bean 实现了 BeanNameAware、BeanClassLoaderAware 或 BeanFactoryAware 接口，在这里回调，
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);是BeanPostProcessor 的 postProcessBeforeInitialization 回调
invokeInitMethods(beanName, wrappedBean, mbd);是处理 bean 中定义的 init-method，或者如果 bean 实现了 InitializingBean 接口，调用 afterPropertiesSet() 方法
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName); 这个是BeanPostProcessor 的 postProcessAfterInitialization 回调
好了。到这里，那么IOC的整个过程就完成了，包括容器初始化，bean的初始化、实例化、属性注入，全部跟着源码走了一遍，看起来真复杂，总结一下这个过程吧：

Application加载xml
AbstractApplicationContext的refresh函数载入Bean定义过程
AbstractApplicationContext子类的refreshBeanFactory()方法
AbstractRefreshableApplicationContext子类的loadBeanDefinitions方法
AbstractBeanDefinitionReader读取Bean定义资源
资源加载器获取要读入的资源
XmlBeanDefinitionReader加载Bean定义资源
DocumentLoader将Bean定义资源转换为Document对象
XmlBeanDefinitionReader解析载入的Bean定义资源文件
DefaultBeanDefinitionDocumentReader对Bean定义的Document对象解析
BeanDefinitionParserDelegate解析Bean定义资源文件中的元素
BeanDefinitionParserDelegate解析元素
解析元素的子元素
解析子元素
解析过后的BeanDefinition在IoC容器中的注册（IoC容器的初始化结束）
DefaultListableBeanFactory向IoC容器注册解析后的BeanDefinition（依赖注入开始）
AbstractBeanFactory通过getBean向IoC容器获取被管理的Bean
AbstractAutowireCapableBeanFactory创建Bean实例对象
createBeanInstance方法创建Bean的java实例对象
SimpleInstantiationStrategy类使用默认的无参构造方法创建Bean实例化对象
populateBean方法对Bean属性的依赖注入
BeanDefinitionValueResolver解析属性值
BeanWrapperImpl对Bean属性的依赖注入

转自：https://blog.csdn.net/qq_34203492/article/details/83865450

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